1. Field of the Invention
The present invention relates to an image forming device, and more specifically, relates to an image forming device that optically forms an image of the object by a zoom lens system and outputs it as an electric signal by capturing the image by an image sensor, above all, an image forming device having a high-magnification and small-size zoom lens system.
2. Description of the Related Art
In recent years, digital cameras have been popularized that convert an optical image into an electric signal by using an image sensor such as a CCD (charge coupled device) instead of silver halide film, convert the data into digital form and record or transfer the digital data. Such digital cameras are provided with an image forming device having a taking lens system and an image sensor.
Since the number of pixels of image sensors tends to increase year by year, taking lens systems are required to have high optical performance corresponding to the increased number of pixels of the image sensors. Above all, a compact image forming device is desired that is provided with a zoom lens system capable of varying the magnification over a wide range without degrading the image quality.
Further, in recent years, because of improvements in the image processing capability of semiconductor devices and the like, an image forming device having a zoom function has come to be incorporated in or externally attached to personal computers, mobile computers, mobile telephones, PDAs (personal digital assistants) and the like, and this spurs the demand for high-performance and compact image forming devices.
For size reduction of the image forming device, size reduction of the zoom lens system provided therein is required, and as zoom lens systems aimed at size reduction, for example, so-called three-unit zoom lens systems of negative, positive, positive configuration comprising three units of, from the object side, a first lens unit having negative optical power, a second lens unit having positive optical power and a third lens unit having positive optical power have been proposed in large numbers. This type of zoom lens systems have characteristics such that they are small in the number of lens elements, have a comparatively simple movement mechanism and is suitable for size reduction.
An example thereof is a zoom lens system described in Japanese Laid-Open Patent Application No. 2002-14284. In the zoom lens system described in this document, a meniscus configuration is adopted to a negative lens element in the second lens unit to thereby reduce the thickness of the second lens unit, which enables reduction in the overall length in the collapsed condition. Moreover, although the performance is comparatively high, the number of lens units moved for changing the focal length is small and the movement mechanism is comparatively simple. Thus, this zoom lens system is suitable for compact cameras.
However, in the zoom lens system described in the document, the magnification variation ratio is at most twice even if the second lens unit includes three lens elements, which cannot be said to be sufficient. To achieve a higher magnification variation ratio, it is necessary to increase the number of lens elements of the second lens unit.